The present invention relates generally to an adaptive attenuation circuit and, more particularly, to an adaptive attenuation circuit for adaptively attenuating an alternating differential voltage produced by a magnetic or variable reluctance sensor in response to rotation of a wheel, while maintaining noise immunity.
Inductive magnetic sensors are commonly employed for automotive applications and the like to provide timing signals which enable the determination of position and speed of a rotating wheel. For example, specific applications may include the determination of engine crankshaft position and speed (i.e., RPM) and the determination of wheel speed for anti-lock braking systems. Inductive magnetic sensors generally used for these types of applications are commonly referred to as variable reluctance sensors.
The variable reluctance sensor is generally located adjacent to a rotating wheel which typically has a plurality of circumferentially spaced slots formed therein. The sensor has an inductive magnetic pick-up that is generally made up of a pickup coil wound on a permanent magnetic core. As the wheel rotates relative to the pick-up coil, an alternating voltage is generated in the pick-up coil when the slots on the wheel travel past the sensor. The alternating voltage must then be correctly decoded to recognize high or positive voltage levels. The frequency of the alternating voltage is then determined to achieve rotational speed information about the wheel.
The alternating voltage that is produced with the variable reluctance sensor has peak voltages that generally vary in amplitude according to the rotational speed of the wheel. In a number of automotive applications, the amplitude of the peak voltage may vary from approximately 250 millivolts (mV) at low end speeds to over 250 volts (V) at higher rotating speeds. However, the sensor output is usually fed to a processing module or other control device that is designed to operate within a more limited voltage range. For instance, automotive processing modules are commonly designed with 5 volts CMOS transistors in order to accommodate size and power constraints. For a 5-volt processing module, the input signal may not exceed 5 volts in order to protect the circuitry. Therefore, in order to accommodate a 0-5 Volt range, the sensor output voltage must be properly attenuated when necessary.
In the past, one problem that has remained with some approaches has involved the inability to achieve a limited input voltage without sacrificing noise immunity and accuracy. For example, one approach to limiting the voltage suggests clipping the sensor output voltage at the rails (i.e., ground and 5 volts) and then using the clipped voltage to obtain the frequency information. However, this voltage clipping approach is very susceptible to noise interference, especially at higher speeds where the noise of the signal can approach the 5-volt limit.
Another approach for attenuating the output voltage of a variable reluctance sensor is discussed in U.S. Pat. No. 5,144,233 issued to Christenson et al. and entitled "Crankshaft Angular Position Voltage Developing Apparatus Having Adaptive Control and Diode Control." This approach uses a resistive divider network which is controlled by the forward voltage of a diode. The above-referenced approach has the variable reluctance sensor connected in a single-ended configuration with one end of the pick-up coil connected to ground. In addition, the circuit is capable of swinging to the level of the battery. This single-ended approach operates such that when the input voltage becomes high enough in amplitude to forward bias the diode, a resistive path is established to create a resistor divider network that attenuates the input voltage. However, this approach does not teach the attenuation of a differential voltage, and may not provide sufficient attenuation control that is necessary to protect the 5-volt gates associated with the processing module.
Another approach uses an RC network to attenuate the input voltage. While conventional RC filtering approaches have provided suitable attenuation for low voltage attenuation, high voltage attenuation applications such as those associated with Direct Ignition System (DIS) have exhibited limited attenuation capability. That is, it is generally difficult to provide substantial voltage attenuation for high voltages and also more difficult to control the attenuation. In addition, the RC filter introduces the propensity of a phase shift between the inputs for a differential voltage, thereby potentially causing significant errors in zero-crossing position determination.
More recently, one approach that uses variable resistance to attenuate the input voltage is disclosed in U.S. Pat. No. 5,450,008, entitled "Adaptive Loading Circuit for a Differential Input Magnetic Wheel Speed Sensor," issued to Good et al., and also disclosed in U.S. Pat. No. 5,510,706, entitled "Differential to Single-Ended Conversion Circuit for a Magnetic Wheel Speed Sensor," issued to Good. The aforementioned U.S. patents are hereby incorporated by reference. The approach disclosed in U.S. Pat. Nos. 5,450,008 and 5,510,706 discuss an adaptive loading circuit that provides variable attenuation by switching binary weighted resistors between input nodes. Included in this approach are multiple resistors and switch pairs which provide voltage attenuation. This approach differentially attenuates the voltage input and is capable of providing sufficient attenuation on high input amplitudes for a 5-volt interface circuit. However, a minimum of three resistors are required for each of the differential inputs. In addition, the switch that is employed is designed to be large so that impedance of the voltage divider is dominated by the resistor, which makes for a large area. Further, the input resistance must vary from 1 to 64 as the switch area varies from 64 to 1. This range of values can make it difficult to match various attenuation stages and also adds to increased circuit area.
Accordingly, it is one object of the present invention to provide adaptive attenuation of an alternating differential voltage that is produced by a magnetic sensor in response to the rotation of a wheel. It is a further object of the present invention to provide for a variable attenuation circuit which is capable of handling a differential voltage that is generated by a variable reluctance sensor, with a minimum circuit area, minimum external circuitry and further with minimal errors and maximum noise immunity.